Example
What happens to the pointer on the sensitive A device used to measure electric current. when:
1. The switch is closed and kept closed?
2. The switch is opened and left opened?
3. Explain your answers to 1. and 2.
Answer
1. The needle flicks to the right, and then returns to zero where it remains.
2. The needle flicks to the left, and then returns to zero where it remains.
3. When the switch is closed the current grows to a maximum from zero and there is a changing magnetic field in the primary coil.
This links the secondary coil and induces a current to flow.
The induced current stops when the magnetic field stops changing (i.e. when it has reached full strength), and the needle returns to zero.
It remains there until the magnetic field changes again.
When the switch is opened the current falls to zero and there is a changing magnetic field again in the primary coil.
This links the secondary coil and induces a current to flow – in the opposite direction.
The induced current stops when the magnetic field stops changing (i.e. when it has reached zero), and the needle returns to zero.